Conventionally, it has been known that exhaust recirculation gas (hereinafter, referred to as “EGR gas”) is introduced to an intake line of an internal combustion engine, to reduce an amount of nitrogen oxide (NOx) in exhaust gas. When nitrogen is heated to very high temperatures, it is oxidized into toxic nitrogen oxide. However, the introduction of the EGR gas to the intake line leads to a reduction in an oxygen concentration in intake air and thus gentle combustion of fuel, and also to an increase in a concentration of carbon dioxide which is difficult to heat as compared with oxygen. As a result, a combustion temperature is not likely to be raised, and generation of nitrogen oxide can be suppressed.
It has been also known that, in a case where a tumble control valve is provided in the intake line, when the intake line is narrowed by the tumble control valve during idling or the like where an intake air stream is slow, an intake air speed is increased and generation of a tumble flow (vortex) in the cylinder, as well as atomization of the fuel, is promoted. It is said that, as a result, a mixing rate of the fuel and the intake air becomes high and thus combustion efficiency of the internal combustion engine is enhanced.
For example, there can be mentioned a technique in which a cylinder head of an engine is provided with the tumble control valve, and an EGR opening is formed somewhat downstream of the tumble control valve (see Patent Documents 1 and 2). According to this technique, while the effect of the EGR gas and the effect of the tumble control valve as described above are obtained, foreign materials, such as carbon, contained in the EGR gas is prevented from attaching to the tumble control valve. Therefore, an abnormal operation of the tumble control valve, which may otherwise be caused by an increase in a rotational resistance, is prevented. In addition, carbon and the like does not attach to an inner wall of the intake line around the tumble control valve, and a strength of the tumble flow does not become unstable, which may otherwise be caused by a change in an amount of the intake air leaking from the valve upon the generation of the tumble flow. Therefore, a reduction in the combustion efficiency of the engine can be suppressed.
In addition, according to the technique of Patent Document 1, by arranging an EGR inlet in the vicinity of the tumble control valve, the EGR gas is carried about by the tumble flow and effectively flows into the cylinder. It is said that, with this confirmation, the EGR gas is present in the cylinder in such a manner that it surrounds a mixed gas of the intake air and the fuel, and because of a heat insulating effect of the EGR gas, a cooling loss of the combustion temperature can be suppressed.